We are surrounded in everyday life by yield stress fluids. Simply stated, yield stress fluids remain stationary until a sufficient stress is placed on the fluid at which point the fluid will flow. It can be thought of as the initial resistance to flow under stress and is also referred to as yield value. Yield stress is a measurable quantity similar to, but not dependent on viscosity. While a certain rheology modifier may thicken or enhance the viscosity of a composition in which it is included, it does not necessarily have desirable yield stress properties.
A desirable yield stress property is critical to achieving certain physical and aesthetic characteristics in a liquid medium, such as the indefinite suspension of particles, insoluble liquid droplets, or the stabilization of gas bubbles within a liquid medium. Particles dispersed in a liquid medium will remain suspended if the yield stress (yield value) of the medium is sufficient to overcome the effect of gravity or buoyancy on those particles. Insoluble liquid droplets can be prevented from rising and coalescing and gas bubbles can be suspended and uniformly distributed in a liquid medium using yield value as a formulating tool. An example of a yield stress fluid is a micro-gel rheology modifier which is used generally to adjust or modify the rheological properties of aqueous compositions. Such properties include, without limitation, viscosity, flow rate, stability to viscosity change over time, and the ability to suspend particles for indefinite periods of time. They are useful in a number of consumer and industrial applications. An important consumer application includes their use in the formulation of personal care products such as body washes, skin creams, toothpastes, shampoos, hair gels and other cosmetics. In industrial applications, they are useful as subterranean treatment fluids in the oil and gas industry as a component in drilling and fracturing fluids. Typically, they comprise chemically crosslinked polymers having a pH-responsive functionality that is either base or acid sensitive. The polymers may be mixed with other ingredients in a formulation and then neutralized by the addition of a neutralization agent such as an acid or a base. Acid sensitive thickeners are activated upon contact with an acidic agent, while base-sensitive thickeners are activated upon contact with an alkaline agent. Upon neutralization, the polymers swell significantly to form a randomly close-packed (RCP) jammed network of swollen cross-linked micro-gel particles imparting a desired rheological profile, i.e., yield stress, elastic modulus, and viscosity, as well as optical clarity to the formulation.
These types of rheology modifiers are well known in the art. For example, U.S. Pat. Nos. 2,798,053; 2,858,281; 3,032,538; and 4,758,641 describe cross-linked carboxylic acid polymers based on acrylic acid, maleic acid, itaconic acid or methacrylic acid monomers. U.S. Pat. No. 6,635,702 describes crosslinked alkali-swellable acrylate copolymers comprising one or more carboxylic acid monomers and one or more non-acid vinyl monomers. U.S. Pat. No. 7,378,479 discloses a crosslinked acid-swellable polymer containing at least one basic amino substituent that is cationic at low pH, at least one hydrophobically modified polyoxyalkylene substituent derived from an associative vinyl monomer, and at least one polyoxyalkylene substituent derived from a semihydrophobic vinyl surfactant monomer. A key feature of these pH-responsive micro-gels is the very large increase in diameter (or size) of individual cross-linked polymer particles upon neutralization. The high swelling efficiency allows formulators to achieve the desired yield stress and viscosity using relatively small amounts of polymer resulting in low cost-in-use. Dalmont, Pinprayoon and Saunders (Langmuir vol. 24, page 2834, 2008) show that individual particles in a micro-gel dispersion of a copolymer of ethyl acrylate, and methacrylic acid cross-linked with butanediol diacrylate increase in diameter by at least a factor of 3 upon pH-activation or neutralization. The level of swelling causes an increase in volume fraction of at least 27 (33). A jammed network is achieved upon neutralization (or activation) with a relatively low concentration of polymer (less than 3 wt. %).
Although pH-responsive micro-gels provide yield stress fluids with the high efficiency that is desired by the formulator, they suffer from a major disadvantage. Rheological properties are not uniform across a broad range in pH and show sharp changes as a function of pH. To overcome these difficulties, various non-ionic thickeners have been proposed. U.S. Pat. No. 4,722,962 describes non-ionic associative thickeners comprising a water-soluble monoethylenically unsaturated monomer and a non-ionic urethane monomer. These polymers provide increases in viscosity or thickening of aqueous formulations that is relatively independent of pH but the polymers are not cross-linked and the purely associative interactions do not create a yield stress.
In addition to pH-responsive micro-gels, temperature-responsive micro-gels are known in the art. Senff and Richtering (Journal of Chemical Physics, vol. 111, page 1705, 1999) describe the change in size of non-ionic chemically cross-linked poly(N-isopropylacrylamide) (PNIPAM) micro-gel particles as a function of temperature. The particles swell by almost a factor of 2.5 in diameter (15 times in terms of volume fraction) when the temperature is reduced from 35° C. to 10° C. Although this represents a significant degree of swelling, the use of temperature to activate micro-gels is undesirable. A method of activation is needed that enables switching from a free-flowing suspension to a jammed yield stress fluid under ambient conditions.
Wu and Zhou (Journal of Polymer Science: Part B: Polymer Physics, vol. 34, page 1597, 1996) describe the effect of surfactant on swelling of chemically cross-linked PNIPAM homo-polymer micro-gel particles in water. The use of surfactants to activate micro-gels is attractive because many formulations contain surfactants as co-ingredients. However, the efficiency of swelling reported by Wu and Zhou is extremely low. The anionic surfactant sodium dodecyl (lauryl) sulfate increases the size of cross-linked PNIPAM particles by only a factor of 1.4 at room temperature. Furthermore, Wu and Zhou do not teach how to create a shear thinning yield stress fluid with high optical clarity.
Another inherent drawback with PNIPAM micro-gels is the low yield of polymer solids attained at the conclusion of synthesis. Thorne, Vine and Snowden (Colloid Polymer Science, vol. 289, page 642, 2011) report that PNIPAM prepared by surfactant free emulsion polymerization (SFEP) results in a solids concentration of 0.5% (w/v). Typical emulsion polymerized commercially available micro-gel latexes contain at least about 28% (w/v) and higher solids levels.
Hidi, Napper and Sangster (Macromolecules, vol. 28, page 6042, 1995) describe the effect of surfactant on swelling of poly(vinyl acetate) homopolymer micro-gels in water. For micro-gels that are not cross-linked they report an increase in diameter by a factor of 3 to 4 corresponding to a 30 to 60 fold change in volume of the original particles in the presence of sodium dodecyl (lauryl) sulfate. However, swelling is drastically reduced for cross-linked particles. In this case, they observe an increase in diameter by only a factor of 1.4. Once again, Hidi, Napper and Sangster do not teach how to create a shear thinning yield stress fluid with high optical clarity.
Apart from providing the necessary rheology profiles, the suspension of solids and/or insoluble materials in a phase stable system is as equally important to a rheology modifier. In drilling for oil and gas, subterranean treatment fluids (e.g., drilling and fracture fluids) are typically modified with gelling agents to provide desired rheological properties. Gelling agents include any substance that is capable of increasing the viscosity of a fluid, for example, by forming a micro-gel. These agents must not only possess desirable rheological properties in terms of fluid flow and pumpability, but must also have the capability to suspend solids under both dynamic and static conditions. During active drilling operations, the drilling fluid must possess sufficient structure to carry the formation cuttings to the surface and also have the necessary shear thinning properties to be pumpable. During non-drilling periods, the drilling fluid may remain stationary in the bore hole for hours or even days at a time. During this period, settling of entrained solids can be problematic if the fluid does not have enough structure to support both large and small particulate matter.
Fracturing is used to boost the production of hydrocarbons such as petroleum or natural gas from subterranean formations. In this process, a fracturing fluid containing a gelling agent is injected through a wellbore and forced against the formation strata by high pressure sufficient to cause the strata to crack and fracture thereby liberating the hydrocarbon trapped in the formation. The fracturing fluid also carries a proppant to the fracture site. Proppant particles remain in the fracture thereby “propping” the fracture open when the well is in production. The proppant material is typically selected from sand, sintered bauxite, glass balls, polystyrene beads, and the like. Whereas sufficient rheological properties are important in treatment fluids used in fracturing, satisfactory suspending ability is necessary for the transport of the proppant materials to the fracture site within the formation.
Conditions are harsh within a subterranean formation and a gelling agent must be stable to variations in temperature, brackish environments, wide ranges of pH, and changes in shear forces.
Various problems have been encountered with subterranean treatment fluids in oil field applications, including the lack of thermal stability of the gel upon exposure to varying temperatures and pH, as well as high shear conditions. This can result in changes in the rheological properties of the gel which can ultimately affect the ability of the fluid to suspend bore hole cuttings and or proppant materials. If particulate materials are prematurely lost from the treatment fluid, it can have a detrimental effect on the drilling and development of the formation. Furthermore, gel instability can result in higher loss of fluid into the formation thereby diminishing the efficiency of the operation.
Personal care compositions which can suspend particles and/or other water insoluble materials are very desirable. These materials impart or contribute to a variety of user benefits including but not limited to exfoliation, visual aesthetics, and/or the encapsulation and release of beneficial agents upon use. The suspension of particulate and insoluble materials as active and aesthetic agents in personal care compositions is becoming increasingly popular with formulators. Typically, particles are suspended in personal care compositions using structuring systems such as acrylate polymers, structuring gums (e.g., xanthan gum), starch, agar, hydroxyl alkyl cellulose, etc. However, the addition of beads or particles to personal care compositions tends to be problematic. For example, one problem is that particles or insoluble materials very frequently tend to be of a different density than the continuous phase of the composition to which they are added. This mismatch in the density can lead to separation of the particles from the continuous phase and a lack of overall product stability. In one aspect, when added particles are less dense than that of the composition continuous phase, the particles tend to rise to the top of such phase (“creaming”). In another aspect, when the added particles have a density greater than that of the continuous phase, the particles tend to gravitate to the bottom of such phase (“settling”). When large particles are desired to be suspended (e.g., polyethylene particles, guar beads, etc.), the level of polymer used is typically increased to provide increased structure for suspended beads. A consequence of thickening a liquid to provide structure for suspended beads causes a significant increase in liquid viscosity and a corresponding decrease in pourability, a property which is not always desirable. Highly viscous products are typically difficult to apply and rinse away, especially if the shear thinning profile of the viscosity building agent is deficient. High viscosities can also adversely affect packaging, dispensing, dissolution, and the foaming and sensory properties of the product. Moreover, conventionally structured liquids are often opaque or turbid thereby obscuring the suspended beads from the consumer, which adversely affects the aesthetic appeal of the product.
Many common thickeners such as xanthan gum, carboxymethylcellulose (CMC), carrageenan, and acrylic acid homopolymers and copolymers are anionic and therefore, can react with the cationic surfactants and cause precipitation of the cationic and thickener or reduce the efficacy of the cationic surfactant. Non-ionic thickeners such as hydroxyethylcellulose (HEC) and hydroxypropylmethylcellulose (HPMC) can provide viscosity in cationic systems, however, very little suspension properties are imparted to the fluid. Cationic thickeners such as Polyquaternium-10 (cationically modified HEC) and cationic guar provide thickening in cationic systems but not suspension. Some acrylic polymers are effective at thickening cationic systems but they can be limited by pH, require high concentrations, have high cost-in-use, and often have narrow limits of compatibility with the cationic materials.
Anionic surfactants are often used as detersive agents in cleansers and cleaning products because of their excellent cleaning and foaming properties. Exemplary anionic surfactants traditionally utilized in these formulations include, for example, alkyl sulfates and alkyl benzene sulfonates. While the anionic surfactants and, in particular, the anionic sulfates and sulfonates are efficient detersive agents, they are severe ocular irritants and capable of causing mild to moderate dermal irritation to some sensitized persons. Accordingly, it has become increasingly important to consumers that aqueous cleansing compositions be mild in that they do not irritate the eyes and skin when in use. Manufacturers are striving to provide mild cleansing products that also incorporate insoluble benefit and/or aesthetic agents that require stable suspension. It is known that the irritation caused by anionic sulfates and sulfonates can be reduced by utilizing the ethoxylated forms thereof. While ethoxylated surfactants may mitigate ocular and skin irritation in compositions in which they are included, a major problem in using these surfactants is that it is difficult to obtain desirable yield stress properties in an ethoxylated system.
U.S. Pat. No. 5,139,770 describes the use of crosslinked homopolymers of vinyl pyrrolidone in surfactant containing formulations such as conditioning shampoo to obtain relatively high viscosities. However, the patent does not teach how to create a yield stress fluid with high optical clarity that is also shear thinning.
U.S. Pat. No. 5,663,258 describes the preparation of crosslinked copolymers of vinyl pyrrolidone/vinyl acetate. High viscosities are obtained when the polymer is combined with water but there is no teaching about using the polymer to create a yield stress fluid that is activated by surfactant.
U.S. Pat. No. 6,645,476 discloses a water soluble polymer prepared from the free radical polymerization of a hydrophobically modified ethoxylated macromer in combination with a copolymerizable second monomer selected from unsaturated acids and their salts and/or a myriad of other monomers including N-vinyl lactams and vinyl acetate. Preferred polymers are crosslinked and are polymerized from hydrophobically modified ethoxylated macromers in combination with neutralized 2-acrylamido-2-methylpropanesulfonic acid. The viscosities of 1% aqueous solutions of the polymer preferably range from 20,000 mPa·s to 100,000 mPa·s. There is no teaching of a surfactant activated polymer devoid of hydrophobically modified ethoxylated macromer repeating units providing a yield stress fluid exhibiting good suspension properties without a substantial increase in viscosity.
There remains a challenge to not only demonstrate the ability to effectively suspend particles within stable micro-gel containing compositions, but also exhibit desirable mildness, desirable rheology profiles, clarity and aesthetic characteristics across a wide range of temperature and pH conditions at low polymer usage levels. Accordingly, there is a need for a yield stress fluid based on polymer micro-gel particles wherein the concentration of polymer is no more than 5 wt. % based on the weight of the composition in which it is included and having yield stress value of at least 0.1 Pa, wherein the yield stress, elastic modulus and optical clarity are substantially independent of pH. There is also a need to provide yield stress fluids formulated with mild surfactants such as, for example, surfactants containing ethylene oxide moieties.